HERE'S A SUMMARY OF ME, WHAT IS RECOMMENDED (and thus what I'm doing) BASED ON FREDDD'S INFORMATION and more:
For ME, it's very recent issue, in that it was only last Fall of 2016 that I was seeing rising B12 at 1500 (while on methyl B12) and rising iron (while on NO iron supplementation). By January of 2017, my B12 was "over 2000" (while on a combo of hydroxy B12/adenosyl B12) and my iron was in the upper 160s, which is WAY too high for a woman. I was also manifesting symptoms of a functional deficiency of B12---numbness in my legs and little fingers. I then moved to Hydroxy B12, which got rid of the deficiency symptoms...BUT...I was quickly to find out from Freddd that it was NOT the way to go using Hydroxy B12, even if it helped me lose those deficiency symptoms.
First, his brilliant info taught me that we NEED methyl B12--he said start at 1000 mcg a day. It affects over 600 functions in our body, and it should be the main B12 in our cells. He said to make one's way up to 15,000-20,000 mcg over many weeks or months I don't know yet if that will be true for me. I've only gone up to 2000 mcg and am work in progress on all this. (I also stopped the B12 today since my blood B12 is still quite high i.e. over 2000, and am instead raising folate. More on that later) And by the way, I have six ++ FUT2 which can affect absorption of B12. hmmmm
We also need some adenosyl B12--he said to take twice a week instead of methyl those two days, but I've also noticed him saying once a week in another place. He also said to wait a week before starting. That's the B12 that resides in our mitochondria. (That really struck me as I have a mito issue, too, probably from mold damage, and later high copper. So I could see how important that was for me.) I've seen to start on 500 mcg and work one's way up to 30,000-40,000 mcg (I also saw up to 50,000 in another comment)...but again, don't know if I would need that much yet. Still work in progress.
Now WHY would I want to get back on methyl B12 and adenosyl B12 when I know for a fact that my serum B12 was rising on both, and I was having deficiency symptoms?? This where l-folate comes in...and you want to be sure and use an active one, like Metafolin, of any of these: L-5-MTHF or L-5-Methyltetrahydrofolate or 6(S)-L-MTHF or 6(S)-L-Methyltetrahydrofolate or (6S)-5-methyltetrahydrofolic acid. Interestingly in early March 2017, I did a Spectracell test, and guess what was "borderline deficient"? Folate (and other B's) !!! And guess what else?? I also am ++ and+- for MTRR, which is a folate SNP, resulting in poor methylation of B12. I just clearly need way more folate than I've been taking all this time...and perhaps THAT is one reason my methylation started working poorly. So I'm now up to 2000 mcg and will keep going up to see if that will help my B12 break down for use!! I retested my B12 and on 1600 mg folate, it's STILL "over 2000".
Also important is L-carnitine fumerate--the fumerate is an important version apparently. Carnitine works with the adenosyl and the two are important combinations for energy and mood. It's recommended to start low, such as 250 mg and make your way up to approx 1000 mg a day. I was already on it.
Finally, since using all the above will help methylation to kick in, your potassium requirements will go up, up for your cell division!! Without extra potassium, they say you will notice this by day 3. For me, it was day 5, and I noticed increasing fatigue. So I got on 4 cups of V8 juice a day to equal 2000 mg of potassium (500 mg a cup). Really helped. I have stayed on it for over two weeks now. I've also read that increasing folate will increase the need for potassium, too.
It's also mentioned to be on on SAM-e because it makes the methylcobalamin more effective. I'm on 400 mcg and was before I even started all the above. And to use TMG, which enhances the effects of SAM-e, Methyl B12 and L-Carnitine. I'm taking SO MANY supps right now due to various issues including mito dysfunction that I'm not on TMG.
I am also bringing up other B's which Spectracell said were borderline deficient, including biotin and b6 and other nutrients. I highly recommend Spectracell testing, but be prepared it's expensive.